1. Field of the Invention
The present invention relates a non-aqueous electrolyte capable of providing a lithium secondary battery superior in the cycle characteristics and the battery characteristics such as the electric capacity and storage characteristics and a. lithium secondary battery using the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have been widely used as sources of power for driving compact electronic devices etc. A lithium secondary battery is mainly composed of a cathode, a non-aqueous electrolyte and an anode. In particular, a lithium secondary battery using a lithium complex oxide such as LiCoO2 as a cathode and a carbonaceous material or lithium metal as an anode is suitably being used. Further, as the non-aqueous electrolyte for such a lithium secondary battery, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) and an electrolyte salt such as a lithium salt are suitably used.
However, a secondary battery having more superior characteristics in the cycle characteristics of the battery and the battery characteristics such as the electric capacity has been required.
In a lithium secondary battery using, for example, LiCoO2, LiMn2O4, LiNiO2, as the cathode, the solvent in the non-aqueous electrolyte is locally partially oxidized and decomposed at the time of charging. The decomposed products inhibit the desirable electrochemical reaction of the battery, and therefore, causes a decrease in the battery characteristics. This is believed due to the electrochemical oxidation of the solvent at the interface between the cathodic material and the non-aqueous electrolyte.
Further, in a lithium secondary battery using, for example, a highly crystallized carbonaceous material such as natural graphite or artificial graphite as the anode, the solvent in the non-aqueous electrolyte is reduced and decomposed at the anode surface at the time of charging. Furthermore, in EC, which is generally broadly used as a solvent for a non-aqueous electrolyte, partial reduction and decomposition occur during the repeated charging and discharging and, therefore, the battery performance is decreased. Thus, the cycle characteristics of the battery and the battery characteristics such as the electric capacity are not necessarily satisfactory.
The objects of the present invention are to solve the above-mentioned problems relating to the non-aqueous electrolyte for a lithium secondary battery and to provide a non-aqueous electrolyte for a lithium secondary battery capable of providing a lithium secondary battery superior in the cycle characteristics of the battery and the battery characteristics such as the electric capacity and storage characteristics in the charging state and also to provide a lithium secondary battery using the same.
In accordance with the present invention, there is provided a non-aqueous electrolyte comprising a non-aqueous solvent, an electrolyte salt dissolved therein and a tert-butylbenzene derivative having the formula (I): 
wherein R1, R2, R3, R4 and R5 independently represent a hydrogen atom or C1 to C12 hydrocarbon group.
In accordance with the present invention, there is also provided a lithium secondary battery comprising a cathode, an anode, and a non-aqueous electrolyte comprising a non-aqueous solvent, an electrolyte salt dissolved therein and a tert-butylbenzene derivative having the formula (I): 
wherein R1, R2, R3, R4 and R5 independently represent a hydrogen atom or C1 to C12 hydrocarbon group.
The non-aqueous electrolyte of the present invention is used as a component of a lithium secondary battery. The components of the lithium secondary battery other than the non-aqueous electrolyte are not particularly limited. Various components generally used in the past may be used.
In this specification and in the claims which follow, the singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
In the tert-butylbenzene derivative having the above formula (I) contained in the non-aqueous electrolyte containing the non-aqueous solvent and the electrolyte salt dissolved therein, R1, R2, R3, R4 and R5 are independently a hydrogen atom; a linear alkyl group such as a methyl group, ethyl group, propyl group and butyl group; or a branched alkyl group such as an isopropyl group, isobutyl group, sec-butyl group, and tert-butyl group. Further, it may be a C3 to C6 cycloalkyl group such as a cyclopropyl group and cyclohexyl group. Further, it may be a phenyl group, benzyl group, or an alkyl-group substituted phenyl group or benzyl group such as a tolyl group, tert-butylphenyl group, or tert-butyl benzyl group. It preferably has a C1 to C12 hydrocarbon group.
As specific examples of the tert-butyl benzene derivative having the above formula (I), for example, tert-butylbenzene (i.e., R1xe2x95x90R2xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 2-tert-butyltoluene (i.e., R1=methyl group, R2xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 3-tert-butyltoluene (i.e., R2=methyl group, R1xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-tert-butyltoluene (i.e., R3=methyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 1-(tert-butyl)-2-ethylbenzene (i.e., R1=ethyl group, R2xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 1-(tert-butyl)-3-ethylbenzene (i.e., R2=ethyl group, R1xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 1-(tert-butyl)-4-ethylbenzene(i.e., R3=ethyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 3-tert-butyl-o-xylene(i.e., R1xe2x95x90R2=methyl group, R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-tert-butyl-o-xylene(i.e., R2xe2x95x90R3=methyl group, R1xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-tert-butyl-m-xylene(i.e., R1xe2x95x90R3=methyl group, R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 5-tert-butyl-m-xylene(i.e., R2xe2x95x90R4=methyl group, R1xe2x95x90R3xe2x95x90R5=hydrogen atom), 2-tert-butyl-p-xylene(i.e., R1xe2x95x90R4=methyl group, R2xe2x95x90R3xe2x95x90R5=hydrogen atom), 3-iso-propyl-1-tert-butylbenzene (i.e., R2=iso-propyl group, R1xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-iso-propyl-1-tert-butylbenzene(i.e., R3=iso-propyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-n-butyl-1-tert-butylbenzene(i.e., R3=n-butyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-iso-butyl-1-tert-butylbenzene(i.e., R3=iso-butyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-sec-butyl-1-tert-butylbenzene(i.e., R3=sec-butyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 3-cyclohexyl-1-tert-butylbenzene(i.e., R2=cyclohexyl group, R1xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 4-cyclohexyl-1-tert-butylbenzene(i.e., R3=cyclohexyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 4,4xe2x80x2-di-tert-butyldiphenylmethane(i.e., R3=4-tert-butylbenzyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 4,4xe2x80x2-di-tert-butylbiphenyl(i.e., R3=4-tert-butylphenyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 1,3-di-tert-butylbenzene(i.e., R2=tert-butyl group, R1xe2x95x90R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 1,4-di-tert-butylbenzene(i.e., R3=tert-butyl group, R1xe2x95x90R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 1,2,4-tri-tert-butylbenzene(i.e., R1xe2x95x90R3=tert-butyl group, R2xe2x95x90R4xe2x95x90R5=hydrogen atom), 1,2,3-tri-tert-butylbenzene(i.e., R1xe2x95x90R2=tert-butyl group, R3xe2x95x90R4xe2x95x90R5=hydrogen atom), 1,3,5-tri-tert-butylbenzene(i.e., R2xe2x95x90R4=tert-butyl group, R1xe2x95x90R3xe2x95x90R5=hydrogen atom), 1,2,3,5-tetra-tert-butylbenzene(i.e., R1xe2x95x90R2xe2x95x90R4=tert-butyl group, R3xe2x95x90R5=hydrogen atom), 1,2,3,4-tetra-tert-butylbenzene(i.e., R1xe2x95x90R2xe2x95x90R3=tert-butyl group, R4xe2x95x90R5-=hydrogen atom), 1,2,4,5-tetra-tert-butylbenzene(i.e., R1xe2x95x90R3xe2x95x90R4=tert-butyl group, R2xe2x95x90R5=hydrogen atom), 3,5-di-tert-butyltoluene(i.e., R2=methyl, R4=tert-butyl group, R1xe2x95x90R3xe2x95x90R5=hydrogen atom), etc. may be mentioned.
If the content of the tert-butylbenzene derivative having the formula (I) contained in the non-aqueous electrolyte is excessively large, the battery characteristics is sometimes decreased, while if excessively small, the desired sufficient battery characteristics cannot be obtained. Therefore, the content thereof is 0.1 to 20% by weight based upon the weight of the non-aqueous electrolyte, preferably 0.2 to 10% by weight, particularly preferably 0.5 to 5% by weight, in which ranges the cycle characteristics are improved.
The non-aqueous solvent used in the present invention is preferably composed of a high dielectric constant solvent and a low viscosity solvent.
As the high dielectric constant solvent, for example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and vinylene carbonate (VC) may be suitably mentioned. These high dielectric constant solvents may be used alone or in any combination thereof.
As the low viscosity solvent, for example, linear carbonates such as dimethyl carbonate (DMC), methylethyl carbonate (MEC), and diethyl carbonate (DEC); ethers such as tetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane and 1,2-dibutoxyethane; lactones such as xcex3-butyrolactone; nitrites such as acetonitrile; esters such as methyl propionate; and amides such as dimethyl formamide may be mentioned. These low viscosity solvents may be used alone or in any combination thereof.
The high dielectric constant solvent and the low viscosity solvent are freely selected and combined for use. Note that the high dielectric constant solvent and low viscosity solvent are used in a ratio by volume (high dielectric constant solvent:low viscosity solvent) of usually 1:9 to 4:1, preferably 1:4 to 7:3.
As the electrolyte salt used in the present invention, for example, LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiPF3(CF3)3, LiPF3(C2F5)3, LiPF4(C2F5)2, LiPF3(iso-C3F7)3, LiPF5(iso-C3F7), etc. may be mentioned. These electrolyte salt may be used alone or in any combination thereof. These electrolyte salt are normally used in the dissolved form in the non-aqueous solvent at a concentration of 0.1 to 3M, preferably 0.5 to 1.5M.
The non-aqueous electrolyte of the present invention can be obtained, for example, by mixing the above high dielectric constant solvent and low viscosity solvent, dissolving the electrolyte salt therein and dissolving the tert-butylbenzene derivative having the formula (I) therein.
For example, as the cathode active material, a complex metal oxide of at least one metal selected from the group comprising cobalt, manganese, nickel, chrome, iron and vanadium with lithium may be used. As such a complex metal oxide, for example, LiCoO2, LiMn2O4, LiNiO2, LiNi0.8CO0.2O2, etc. may be mentioned. These cathode active materials may be used alone or in any combination thereof.
The cathode is prepared by mixing the cathode active material with a conductive agent such as acetylene black or carbon black, a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), and a solvent to form a cathode paste, then coating the resultant cathode paste on a collector such as aluminum foil or a stainless steel lath, drying and compression molding the same, followed by heat treating at a temperature of about 50 to 250xc2x0 C. for about 2 hours, for example, under a vacuum.
As the anode active material, lithium metal, a lithium alloy, a carbonaceous material having a graphite-like crystalline structure (heat cracked carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic polymer compound sintered bodies, carbon fiber), complex stannous oxides or other substances, which are capable of absorbing and releasing lithium are used. In particular, a carbonaceous material having a graphite-like crystalline structure having a lattice spacing (d002) of the lattice face (002) of 0.335 to 0.340 nm is preferred. These anode active materials may be used alone or in any combination thereof. Note that a powder material such as a carbonaceous material is mixed with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) to make an anode paste. The method for producing an anode is not particularly limited. The anode may be produced by an analogous method as the above production method of the cathode.
The structure of the lithium secondary battery is not particularly limited. A coin battery having a cathode, anode, single-layer or multiple-layer permeable separator and a cylindrical battery, prismatic battery, etc. having a cathode, anode, and roll-type separator may be mentioned as examples. Note that as the separator, a known polyolefin microporous membrane, woven fabric, nonwoven fabric, etc. is used.